The invention relates to a press brake used in particular for bending metal sheets.
An example of a press brake, as known in the prior art, is shown schematically in FIGS. 1 and 2.
It comprises an upper beam 1 placed above a lower beam 2. The latter is a fixed beam, bearing on its ends, while the upper beam 1 is a moving beam and is actuated in a vertical plane by drive members located also at its two ends.
The drive members deliver the force needed to bend the metal sheets or plates.
More specifically, the beams 1, 2 are mounted in a frame formed from two side plates 9a and 9b joined together especially by a bracing beam (not shown).
The upper beam 1 and the lower beam 2 are contained in the same vertical plane and the upper beam slides with respect to the side plates 9a and 9b with the aid of guiding means 8a and 8b consisting, for example, of two hydraulic rams.
Working edges of these two, upper and lower, beams bear a bending punch P and a corresponding die M, respectively.
As may be seen in FIG. 2, the lower part 4 of the lower beam 2 is fastened, by welding or by any other means, at its ends to the side plates 9a and 9b forming the frame of the press brake. FIG. 3 shows a sheet 10 placed on the die M in which a xe2x80x9cVxe2x80x9d, which will allow the bending, is formed. A force F is exerted along the axis of the xe2x80x9cVxe2x80x9d, and at the extreme tip 12 of the punch P, in order to bend the sheet.
The bend angle of a metal plate or sheet depends on the extent of penetration of the punch P into the die M.
A press brake may, in general, carry out three types of bending.
The relative movement of the punch may be stopped at the stage shown in FIG. 4. This represents a first type of bending, called xe2x80x9c3-point air bendingxe2x80x9d.
This type of bending is obtained by limiting the stroke of the beam 1 during the set-up of the machine.
If, on the contrary, the penetration is increased, the sheet 10 descends into the xe2x80x9cVxe2x80x9d up to a limit defined by the bottom of the V (FIG. 5). This represents the technique called xe2x80x9csemi-coiningxe2x80x9d. This technique has furthermore the following characteristics:
the radius Ri of the sheet, or plate, 10, internal to the bent zone, is in general equal to or slightly greater than the thickness of the sheet;
when the pressure of the punch is released, reopening of the bend occurs, due to the residual elasticity of the sheet 10.
Finally, if the force is again increased, the tip 12 penetrates the sheet 10 and xe2x80x9cswagesxe2x80x9d the bend radius (FIG. 5). This represents so-called xe2x80x9ccoiningxe2x80x9d bending which has the following features:
the inside radius Ri is less than the thickness of the sheet; it is determined by the radius of the punch;
the bend angle is equal to that of the xe2x80x9cVxe2x80x9d of the die M and of the punch, the elasticity of the sheet having disappeared.
In the case of 3-point air bending, since the side walls of the bend, of the punch and of the die are never in contact with one another, the shape of the die is of little importance. It may, moreover, be a U.
Compared with bending to the bottom of the xe2x80x9cVxe2x80x9d and coining, air bending is that requiring the least force and the metal remains highly elastic.
These elements mean that this bending shape is the most sensitive to angular variations and requires particular attention in carrying it out.
In particular, in xe2x80x9c3-pointxe2x80x9d bending, experience shows that a mechanical difference of {fraction (1/10)} of a millimeter, measured for example between two 12-tip elements of two punches, results in an angular variation of 2xc2x0 in bending a 2 mm sheet performed in a V of 12 (i.e. 6 times the thickness).
In general, and still in the case of xe2x80x9c3-pointxe2x80x9d bending, using a width corresponding to 8 to 12 times the thickness of the sheet 10 to be bent allows partial bending to be carried out with a tolerance of xc2x11xc2x0.
This is the optimum precision obtained with air bending.
A method making it possible to help in carrying out bending operations with optimum precision consists in using a protractor 16, mounted as illustrated in FIG. 6: the sheet 10 can bear on the arm 18 of the protractor, said arm being mounted on the die M.
When the edge of the sheet 10 is parallel to the arm of the protractor, the pressure on the punch is reduced to the minimum with the aid of the power control so as to allow the sheet to release the elastic bending stress. The angle A of this elasticity is determined with respect to the desired angle indicated by the protractor.
Next, the pressure is increased so as to increase the depth of bending of the elasticity angle, mentioned above (angle A).
The technique of xe2x80x9csemi-coiningxe2x80x9d also results in springback of the sheet. Consequently, tooling with an 88xc2x0 apex angle, for example, is chosen for 90xc2x0 bending. This 88xc2x0 angle may be reduced to 85xc2x0 for thick sheets.
The bending precision, under optimum conditions, allows a tolerance of xc2x130 minutes of angle to be achieved.
The coining bending is that which allows the highest angular precision to be achieved, the elasticity of the sheet being eliminated. However, this type of bending requires it to be possible to increase, during the 2nd phase of the bending, the force applied to the punch so as to bring the sheet edges back onto the side walls of the V of the die. The angle of the tooling is then the desired bend angle. The tools used must therefore be very accurate in order, in turn, to form the sheet to their specific characteristics.
The angular precision obtained with this type of bending may at best be 15 minutes of angle.
Consequently, it is apparent that the question of the precision of a press brake is a critical problem which, in most cases, is difficult to solve.
Moreover the bending precision is all the more difficult to obtain the thinner the sheet 10.
For heavy plate, unlike thin sheet, the imperfections become negligible compared with the unitary penetration for 1xc2x0.
There are also numerical control presses in which an operator enters a desired angle. The control then calculates the penetration and the force that are needed to obtain the desired angle. The calculation is performed with the aid of a known formula or with one developed by the user.
However, this formula can only be an approximation of reality and is not in general applicable to all cases or in the various types of bending, or does not have the same precision in all cases and in the various types of bending.
The problem arises as to how to make the bending machines more precise.
In particular, the problem arises of how to obtain a more precise calculation, or a more precise evaluation or indication, of the bending penetration. Document JP-60-247 415 describes a press brake provided with a means for measuring distances between a lower tool and an upper tool and with a computing means for calculating an effective bend angle of a workpiece as a function of the distance measurements made. The effective bend angle is compared with the bend angle to be attained, and a correction to the descent of the tool is determined. A memory stores information relating to the relationship between the effective bend angle and the bend angle to be attained, and the bend angles and the level of descent of the tool.
The device described in that document involves a step of calculating the effective angle from measured distances and determines a correction to the descent of the tool according to these measured distances.
The precision obtained with this type of device is not satisfactory. This is because the calculation made during the calculation step is necessarily limited in its precision and its validity.
Furthermore, this method does not make a distinction according to the various types of bending carried out. An angle calculated for one measured distance and for one given type of bending is not necessarily valid, or does not necessarily have the same type of precision, for another type of bending.
The subject of the invention is firstly a numerical control system for a bending machine, comprising:
means for inputting, as input data, a desired bend angle and bending conditions or criteria;
means for storing one or more groups of data, each containing operating or bending conditions, a bend angle and at least one penetration depth;
means for searching whether the input data is stored in the memory means, in the same group of data; and
means for transmitting a signal representative of the penetration depth included in said same group of data, or for transmitting a signal for controlling the bending machine according to this penetration depth.
Thus, when the bending conditions and a desired angle, indicated by an operator, exist in the database, the device or the control system recovers the penetration value, stored in the memory means, which corresponds to these bending conditions and this desired angle.
It is therefore possible to carry out a bending operation according to the operating conditions employed, hence improved precision of the bending.
The value of the penetration depth then depends no longer only on a single variable, such as the distance between the lower and upper parts of the press.
The device is particularly advantageous in the case of three-point air bending or V-bottom air bending (semi-coining technique). In fact it is in these bending procedures that the problems of precision are most keenly felt.
According to one particular embodiment, the control device may furthermore include means for searching whether there exists, in the memory means, two groups of data having the same bending conditions as those input by the inputting means, and respective bend angles between which the desired angle lies, and for calculating a penetration depth according to the penetration depths belonging to the two groups of data, respectively.
The calculation of the penetration depth may consist, for example, of an interpolation between the penetration depths contained in the two groups of data. and/or desired when the two aforementioned groups of bending data cannot be found, it is possible to calculate the penetration depth using a predetermined and preprogramed formula.
Advantageously, means may make it possible to modify, in the memory means, at least one parameter from among the bending conditions, the bend angles and the penetration depths.
Thus, the operator is not limited to the values stored in the memory means. Preferably, means are furthermore provided for comparing a measured bend angle with the desired bend angle, and means for correcting the penetration depth if the result of the comparison is that the measured angle is different from the desired angle.
Also preferably, means make it possible to update data in the memory means according to the result of the correction to the penetration depth. Other means may be provided for writing, into the memory means, an additional croup of data containing the input data and the corrected penetration depth. The latter means are used when the input data are not already present in the same group of data stored in the memory means.
The device according to the invention thus has a changing, or dynamic, database which makes it possible to obtain greater precision as and when it is used.
It is thus possible to change the collected data according to the experience gained or to the operation of the machine. None of the machines known at the present time allow such a change. The precision of the machine improves as and when it is used: the more it is used, the more often it encounters various situations (which, statistically, cannot fail to occur) and the more numerous the situations that can be stored in the database.
The subject of t he invention is also a press brake system comprising a control system as described above.
The invention also relates to a numerical control process for a bending machine comprising the following steps:
storing, in memory means, one or more groups of data each containing bending conditions, a bend angle and at least one penetration depth;
receiving, as input data, a desired bend angle and bending conditions or criteria;
searching whether the input data is stored in the memory means, in the same group of data; and
transmitting a signal representative of the penetration depth contained in said same group of data, or a signal for controlling the machine according to this penetration depth.
This process has the same advantages as those described above in relation to the numerical control system according to the invention.